fix(core): generate the image cannot enter text prompt (#12717)

Close [AI-167](https://linear.app/affine-design/issue/AI-167)

![截屏2025-06-05 12.15.49.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/c1afff0e-1197-46dc-ae43-ff7257039509.png)

![截屏2025-06-05 12.14.07.png](https://graphite-user-uploaded-assets-prod.s3.amazonaws.com/sJGviKxfE3Ap685cl5bj/1439b0a7-1cca-4848-aea2-84bc73c536c5.png)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Improved AI panel behavior with explicit modes for input and answer generation, providing a more intuitive user experience when interacting with AI features.

- **Refactor**
  - Streamlined AI panel toggling logic for more consistent and predictable panel states during different actions.

- **Tests**
  - Enhanced AI image generation test to simulate user input and send actions for more accurate end-to-end validation.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/affine-cloud-copilot/e2e/utils/editor-utils.ts

@@ -551,9 +551,11 @@ export class EditorUtils {
       explainImage: this.createAction(page, () =>
         page.getByTestId('action-explain-image').click()
       ),
-      generateImage: this.createAction(page, () =>
-        page.getByTestId('action-generate-image').click()
-      ),
+      generateImage: this.createAction(page, async () => {
+        await page.getByTestId('action-generate-image').click();
+        await page.keyboard.type('generate an image');
+        await page.getByTestId('ai-panel-input-send').click();
+      }),
       generateCaption: this.createAction(page, () =>
         page.getByTestId('action-generate-caption').click()
       ),